1. Field of the Invention
In a first aspect thereof, the present invention relates to a method for manufacturing a belt structure of a green tire for vehicles.
This invention also refers to a plant for manufacturing the belt structure, a method and a plant for manufacturing a belt package of a green tire incorporating the above-mentioned belt structure, as well as a method and a plant for manufacturing a crown structure of a green tire including the above-mentioned belt package.
2. Description of the Related Art
A green tire is essentially constituted by a composite structure obtained by assembling together a plurality of semi-finished products adapted to constitute a carcass comprising at least one airproof liner, two bead wires to which the edges of at least one carcass ply are associated after interposition of an elastomeric filling fixed to the radially outer surface of each bead wire, a belt structure arranged as a crown around the torically shaped carcass, a tread wrapped around the belt structure and a pair of axially opposite sidewalls, radially extending between the bead wires and the tread.
An additional belt layer incorporating a plurality of circumferentially oriented reinforcing cords, otherwise known as zero degree cords, may be inserted between the belt structure and the tread. Preferably, the above-mentioned additional layer is formed by spirally winding around the belt structure a ribbon (or tape) of rubber mixture incorporating a certain number of such cords.
In the following description and in the subsequent claims, the term “belt strip” is used to indicate a continuous strip of rubber-coated fabric provided with reinforcing cords parallel to one another.
In the following description and in the subsequent claims, the term “belt structure” is used to indicate a composite structure including at least a couple of portions of belt strips, obtained by cutting to size said continuous belt strips. Such structure is formed by radially superposing said portions of belt strips, so that the reinforcing cords incorporated in said portions are parallel to one another in each portion of belt strip and inclined with respect to the cords of the adjacent portion, generally in a symmetrical manner with reference to the equatorial plane of the tire. The portions of belt strips cut to size and radially superposed so as to form the belt structure are called in this way because they are obtained by cutting a substantially continuous strip of indefinite length named “belt strip”.
In the following description and in the subsequent claims, the term “belt package” is used to indicate the assembly constituted by a belt structure and by an additional radially outer reinforcing layer incorporating the reinforcing cords oriented circumferentially with respect to the tire. Preferably, said additional layer is manufactured by spirally winding onto said belt structure, in subsequent spires axially placed side by side, a continuous ribbon (tape) of rubber-coated fabric of indefinite length, provided with reinforcing cords made of textile or metallic material, longitudinally arranged with respect to such ribbon.
Finally, in the following description and in the subsequent claims, the term “crown structure” is used to indicate a composite structure constituted by a belt package and by a tread arranged in turn as a crown around the additional layer of reinforcing cords of the belt package.
In the field of tire manufacturing, one of the requirements to be satisfied is notoriously that of imparting to the finished product high quality and long lasting characteristics, achieving at the same time a high production capacity of the manufacturing plant as a whole. In this respect, a critical factor which sharply influences the quality of the finished tire is the quality of the semi-finished products which form the tire structure once they have been assembled together.
The quality of the semi-finished products is in turn linked to the production methods and to the environmental conditions in which the semi-finished products are produced and optionally stored.
According to the prior art, in the manufacturing of the belt package of the green tire, the belt strips and the rubber mixture ribbon (or tape), both produced as a continuous ribbon, are wound around spools to form overlapping spires and are subsequently stored until they have to be used in the production line and assembled on an assembly drum.
A first problem corrected to this type of handling of the semi-finished products is essentially linked to possible variations of the adhesion, of the chemical-physical characteristics and/or of the dimensional characteristics of the semi-finished products, which take place both during the winding onto the spool, during the storing period and during the various handling steps which such semi-finished products undergo before being used in the production line.
During the storing period in which they are kept in spools, in fact, a change of the humidity and temperature conditions of the storing environment from those considered to be optimal may lead to an unwanted variation of the rubber stickiness which, in case of a reduction thereof, creates process problems during the tire manufacture and, in case of an increase thereof, makes the semi-finished product unusable, generally because it becomes impossible to remove the semi-finished product from the spool with an ensuing waste of material.
A second problem connected to the methods of handling and storing the above-mentioned semi-finished products consists in the packing of the radially innermost layers of material in the spool, due to the weight of the material itself, which packing generally determines the appearance of undesired deformations and stresses which influence in turn the quality of the final product in a negative manner. In the worst cases, such packing of the layers may cause the rejection of the material which may no longer be used.
Another problem related to the above-mentioned handling methods of the semi-finished products, in particular of the belt structure, of the additional layer incorporating the zero degree cords and of the tread, consists in the need to carry out a series of cumbersome operations of loading and unloading of the spools in the assembling machine, operations which require both a continuous need for labor intervention and periodical stops, all to the detriment of the quality of the final product and of the productivity of the plant used for manufacturing the green tire.